Arvind Gupta's Toys from Trash (very cool--I'd love to see some of you choose from these)
http://www.arvindguptatoys.com/toys.html

Steve Spagler (This guy's site is set up to sell stuff, but there are some good ideas that don't require purchased items)
http://www.stevespanglerscience.com/

TOPS -- Teach Science With Simple Things website.
You need to click around on this a bit, but at some point you'll see a list that says "More Sample Labs Available" and there is a list of titles you can download for free. They are also trying to sell the activity idea guides, but there's plenty there for free, too.
http://topscience.org/home.html